Conventional lighting systems used in automotive vehicle applications such as headlights, taillights, and active night vision systems utilize an incandescent bulb with a reflector. The light emitted by the incandescent bulb is generally collimated by the reflector. The incandescent bulb may be used to generate light in the visible spectrum for headlight and taillight applications. Active night vision systems typically require near-infrared emissions that are compatible with solid state CCD or CMOS cameras to illuminate the scenery.
Advances in solid state lasers have given rise to thin-sheet lighting systems for use in taillight and active night vision systems. The thin-sheet systems require less space than bulb-and-reflector systems. Furthermore, laser diodes are more energy efficient and reliable than incandescent bulbs. A challenge in thin-sheet lighting systems is to rapidly spread the laser light over a sufficiently wide area to meet spatial illumination and eye safety requirements required under law. Many different approaches have been suggested to expand the point source of laser light uniformly over several square inches of an exterior optical surface.
U.S. Pat. No. 5,791,757, issued to O'Neil et al. on Aug. 11, 1998, discloses a lighting system that uses a uniform thickness thin-sheet optical element. This optical element has a plurality of micro-optical wedges that collimate and direct divergent laser light emitted from multiple fiber optic bundles. Diffractive optical elements are disposed intermediate the thin-sheet and fiber optics to direct the laser light to predetermined regions of the plurality of micro-optical wedges. The diffractive optical elements guide the light to the micro-optical wedges either in a direct path, or by bouncing the light off the exterior side of the thin-sheet opposite the micro-optical wedges. The optical efficiency of this approach could be improved upon if the diffractive optical element could be eliminated. Design complexity could be reduced if each of the micro-optical wedges did not have to be designed to receive the light from a different incident angle.
U.S. Pat. No. 6,422,713 describes a solid optical element that uses total internal reflection to reflect light from various facets. The light is projected into the optical element and projected outwardly by the facets. The optical element may be made of a plastic material. One drawback to this is that some plastic materials absorb light in the infrared range desirable for use in night vision illumination.
Therefore, it would be desirable to provide an optical structure that does not rely on internal reflection for distributing light in the infrared range.